The vertical distribution of volcanic SO2

Elisa Carboni is a postdoctoral scientist at the University of Oxford. She works on developing retrievals of volcanic gas and ash using EO data, and is co-funded by COMET and the SHIVA project.

Tropical eruptions ‘love’ the tropopause

Sulphur dioxide (SO2) plays a crucial role in many atmospheric processes. In the troposphere, it leads to the acidification of rainfall, while in the stratosphere it oxidises to form a sulphuric acid (H2SO4) haze that can affect climate for several years.

Volcanoes contribute around one third of the tropospheric sulphur burden, of which the majority is SO2. Satellites play a crucial role in quantifying volcanic SO2 emissions, but previous measurements have lacked data on the height of these emissions, a key parameter when assessing their effects and lifetime.

The Infrared Atmospheric Sounding Instrument (IASI) on the Metop satellite can however be used to study volcanic emission of SO2 using high-spectral resolution measurements from 1000 to 1200 cm-1 and from 1300 to 1410 cm-1 (the 7.3 and 8.7 μm SO2 bands).

The advantages of the IASI SO2 measurements are that they are not affected by underlying cloud and are consistent (within the retrieved errors) with other measurements (Brewer ground measurements for the column amount of SO2 and CALIPSO for plume altitude).

We applied Carboni et al.’s (2012) scheme to measuring volcanic SO2 amount and altitude for several explosive eruptions between 2008 and 2012 (Carboni et al., in review), showing that the biggest emitter of volcanic SO2 was Nabro (Eritrea), followed by Kasatochi (Aleutian Islands) and Grimsvötn (Iceland).

There is a tendency for volcanic SO2 plumes to reach a point of neutral buoyancy near the tropopause for many of the moderately explosive eruptions observed. This tendency was independent of the maximum amount of SO2 (e.g. 0.2 Tg for Dalafilla (Afar, Ethiopia) compared with 1.6 Tg for Nabro) and of the volcanic explosive index (between 3 and 5). All of the eruptions in the tropics (except Nyamuragira, Congo), reached the tropopause. In the mid latitudes, the eruptions of Eyjafjallajökull, Llaima, Copahue and Etna remained confined to the troposphere.

SO2 vertical distribution for Dalafilla and Nabro volcanic eruption. The colour represents the total mass of SO2 in Tg, dark-red represents values higher than the colour-bar. Every column of the plots come from an IASI map (one every 12 hrs). Red triangles in the bottom line indicate the presence of a fresh plume connected with the volcano.